T cell functional exhaustion in chronic infections and tumors limits effective immunity. Chronic hepatitis B virus (HBV) infection afflicts over 350 million people worldwide and over a million die from chronic HBV infection related diseases. Current treatments for CHB have various limitations and cannot cure most of the CHB patients. Virus-specific CD8+ T cells play an essential role in HBV suppression and clearance through cytolytic and non-cytolytic mechanisms. However, antigen-specific CD8+ T cells in CHB patients have functional exhaustion. In addition, large infiltrates of non-antigen-specific lymphocytes and monocytes in the livers of CHB patients are tightly associated with persistent inflammation and liver damage. These infiltrates of non-antigen-specific cells lead to chronic necroinflammatory liver disease and ultimately hepatocellular carcinoma. Thus, therapeutic approaches restoring exhausted T cells and removing the sequestered non- antigen-specific cells in the liver are necessary steps to achieve a complete cure of CHB. We have investigated CD8+ T cell exhaustion during chronic HBV infection and found that HBV antigen-specific CD8+ T cells are sequestered in the spleens of CHB patients and fail to expand upon antigen stimulation. Furthermore, we found that splenic HBV-specific CD8+ T cells and intrahepatic non-antigen specific CD8+ T cells exhibit reduced expression of S1PR1. We have identified a novel microRNA, miR-720, as a possible regulator of CD8+ T cell exhaustion during chronic HBV infection. MiR-720 expression was elevated in HBV-specific and total CD8+ T cells from CHB patients and its overexpression inhibits proliferation of normal primary human T cells. Furthermore, we have identified the cell proliferation related genes FosB and c-Myc as well as cell migration related gene S1PR1 as miR-720 target genes. Based on these findings, we hypothesize that HBV- induced upregulation of miR-720 in CD8+ T cells is an important determinant in the development of chronic HBV infection. We propose to test whether neutralizing miR-720 can rescue antigen-specific CD8+ T cell function and determine the role of S1PR1 in lymphocyte infiltration and liver damage. Furthermore, we will perform single cell gene expression analysis to elucidate the exhaustion pathways in HBV-specific CD8+ T cells in the context of miR-720 expression and identify the factors that regulate miR-720 expression. Answers to these questions will provide novel insights to the underlying molecular mechanisms of T cell exhaustion and may lead to new immunotherapeutic strategies for the treatment of CHB patients.